I wonder if there is a difference here between circular and point-to-point distances. Imagine you wrap a piece of chain all the way around the sprockets (don’t panic, we’re not going to run the motor with it wrapped, even in our imaginations!) Now form a decagon (a 10-sided polygon), by drawing lines connecting each of the pins (the red shape in the figures). The circumference of that polygon would be Pitch*Teeth = 63.5 mm. Divide by pi (as in the formula from the code) and you have a diameter of a circle that has that same circumference. That circle is smaller than the green circumscribed circle in figure B, and larger than the blue inscribed circle. I haven’t worked out the math, but I suspect the 10.275 mm is the circumference of the green circle.
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